Synthesis of nanometer-sized particles by reverse micelle mediated techniques

ABSTRACT

The present invention relates to a method of producing particles having a particle size of less than 100 nm and surface areas of at least 20 m 2 /g where the particles are free from agglomeration. The method involves synthesizing the particles within an emulsion having a 1-40% water content to form reverse micelles. In particular, the particles formed are metal oxide particles. The particles can be used to oxidize hydrocarbons, particularly methane.

RELATED APPLICATION

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/060,733, filed Apr. 15, 1998, which claims priority to U.S.provisional application serial No. 60/043,321, filed Apr. 15, 1997.

FIELD OF THE INVENTION

[0002] The present invention relates to a method of producing particleshaving a particle size of less than 100 nm and surface areas of at least20 m²/g where the particles are free from agglomeration. The methodinvolves synthesizing the particles within an emulsion having a 1-40%water content to form reverse micelles. In particular, the particlesformed are metal oxide particles.

BACKGROUND OF THE INVENTION

[0003] Nanometer-sized particles have a wide variety of applicationsincluding the areas of magnetic materials, heterogeneous catalysts,toner compositions, and ceramics. Typical techniques to synthesize theseparticles include sol-gel processing and emulsions.

[0004] Emulsions present a unique method to prepare nanometer-sizedparticles. Stable emulsions comprise a continuous phase and adiscontinuous phase which are immiscible, where isolation of thediscontinuous phase is accomplished by the use of surfactants. Thediscontinuous phase constitutes a substantially spherical regime whichcan potentially serve to control particle size. This phase is easilysubjected to disruption, however, due to surface tension that existsbetween the immiscible phases. Thus, there exists a need for methods toobtain unique emulsion compositions that enhance stability of isolatedphases and that yield particles having a desired size and morphology.

DISCUSSION OF RELATED ART

[0005] Metal oxide particles have previously been formed withinmicelles.

[0006] U.S. Pat. No. 5,725,802 relates to a process for the preparationof metal oxide particles. Water-in-oil microemulsions are formed inwhich the oil used is a perfluoropolyether, and metal ions in theaqueous phase are reacted with a gaseous or vapor reactant.

[0007] U.S. Pat. No. 5,670,088 relates to a method for forming mixedmetal oxide particles. A microemulsions is used which includes aperfluoropolyether oil and a perfluoropolyether surfactant. The methodfurther involves mixing one metal in an aqueous phase with a secondmetal in a perfluoropolyether oil phase. The addition of an alkalisolution is accompanied by heating to form the desired oxide.

[0008] U.S. Pat. No. 5,695,901 relates to a method for producingnano-size magnetic iron oxide particles. An iron reactant is containedin a disperse phase, reacted with a basic reactant and subjected to acontrolled oxidation by the addition of a oxygen-containing oxidant.

[0009] European Patent No. EP 0 370 939 relates to a process forproducing ultrafine, magnetic neodymium-iron-boron particles. Theparticles are formed in an emulsion having a discontinuous aqueousphase, comprising an aqueous solution of neodymium-, iron- andboron-containing compounds, which is added to a continuous phase and anionic surfactant to form an emulsion.

SUMMARY OF THE INVENTION

[0010] The present invention provides techniques for making very smallparticles of a variety of materials, small particles of material thatcan be made by the processing method, and methods of use of theseparticles.

[0011] One aspect of the invention provides a method for preparing aparticle. The method involves providing an emulsion having a watercontent of about 1-40% and includes a hydrocarbon and at least onesurfactant. The emulsion forms micelles which comprise a disperseaqueous phase. At least one reactant is added which reacts in and withthe disperse aqueous phase to form a particle having a particle size ofless than about 100 nm where the particle is free from agglomeration.

[0012] Another embodiment of the invention provides a method forpreparing a particle from an emulsion having a water content of about1-40% and includes a hydrocarbon and at least one non-ionic surfactant.The emulsion forms micelles which comprise a disperse aqueous phase. Atleast one reactant is added resulting in a particle having a particlesize of less than about 100 nm where the particle is free fromagglomeration.

[0013] Another aspect of the invention provides a method for coating aparticle within a micelle. Preferred embodiments include coating theparticle with a metal oxide layer.

[0014] In another aspect of the invention, a method is provided fordrying an emulsion including nanometer-sized particles. The methodincludes the steps of forming an emulsion to prepare particles having aparticle size of less than about 1 μm and exposing the emulsion to asupercritical fluid to dry the particles.

[0015] Another method of the invention involves forming an emulsion toprepare particles having a particle size of less than about 1 micron,and exposing the emulsion to a supercritical fluid to dry the particles.

[0016] In another embodiment a method is provided that involveseffecting a reaction in the presence of a reverse emulsion, andproducing a material from the reaction having a particle size of lessthan about 100 nm. The material retains a surface area of at least about100 m²/g when heated to 700° C.

[0017] In another embodiment a method is provided that involvesoxidizing hydrocarbons in the presence of one or more metal oxides. Themetal oxides have an average particle size of less than about 100 nm.

[0018] In another aspect the invention provides a series ofcompositions. In one embodiment a composition is provided that includesa material having an average particle size of less than about 100 nm.The material retains a surface area of at least about 100 m²/g whenheated to 700° C.

[0019] In another embodiment a composition is provided that includes amaterial capable of catalyzing a combustion reaction of a hydrocarbon.The material has a surface area, after exposure to conditions of atleast about 1300° C. for at least about 2 hours, of at least 20 m²/g.

[0020] Other advantages, novel features, and objects of the inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings,which are schematic and which are not intended to be drawn to scale. Inthe figures, each identical or nearly identical component that isillustrated in various figures is represented by a single numeral. Forpurposes of clarity, not every component is labeled in every figure, noris every component of each embodiment of the invention shown whereillustration is not necessary to allow those of ordinary skill in theart to understand the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 depicts a process flowchart outlining the steps for formingan emulsion and synthesizing a particle from the emulsion;

[0022]FIG. 2 shows a transmission electron micrograph (TEM) of Ba—Aloxide microspheres prepared from a 5 wt % water emulsion;

[0023]FIG. 3(a) shows a typical isotherm and (b) shows a desorption poresize distribution;

[0024]FIG. 4 shows an SEM of dried Ba—Al oxide particles havingspherical shape and uniform particle size;

[0025]FIG. 5 shows a plot of the particle size and distribution asobserved in TEM for emulsions of different water contents;

[0026]FIG. 6 shows an optical micrograph of microspheres aged for 72 h;

[0027]FIG. 7 shows the microstructure of Ce—BHA;

[0028]FIG. 8 shows a plot of temperature versus methane conversion forBHA; and

[0029]FIG. 9 shows a plot of temperature versus methane conversion forsurface doped BHA systems.

DETAILED DESCRIPTION

[0030] The present invention provides a series of methods andcompositions involving high surface area particulate material. Smallparticle sizes are provided. In general, processes of the inventioninvolve providing a nanoemulsion and synthesizing particles of materialwithin micelles of the nanoemulsion. The nanoemulsion can be created asdescribed in co-pending, commonly-owned U.S. patent application Ser. No.08/739,509 of Ying, et al., incorporated herein by reference which alsodescribes the use of a cosurfactant. In some cases, the nanoemulsionshould be altered slightly relative to nanoemulsions described in thereferenced application, as described below.

[0031] It is known in the art that small particles can be made withinthe isolated phase of an emulsion. An “emulsion” is a stable mixture ofat least two immiscible liquids. In general, immiscible liquids tend toseparate into two distinct phases. An emulsion is thus stabilized by theaddition of a “surfactant” which functions to reduce surface tensionbetween the at least two immiscible liquids. The emulsions of thepresent invention comprise two immiscible liquids, in particular waterand oil. Emulsions comprise a continuous phase and a disperse phase. Thedisperse phase is the isolated phase stabilized by a surfactant. Thepresent invention involves, in particular, water-in-oil (w/o) emulsionshaving a disperse aqueous phase and a hydrocarbon continuous phase.Alternatively, the isolated disperse aqueous phase is referred to as a“reverse micelle.”

[0032] In typical prior art techniques, the water concentration of theemulsion is quite low and/or the disperse phase of the emulsion includesreverse micelles of relatively large size. The present inventionprovides a water-in-oil emulsion having a high aqueous concentration,small micelle size, and involves formation of particulate materialwithin the disperse aqueous phase. By using these desired surfactantsand reaction conditions, the methods of the present invention provideresults that are surprisingly advantageous since those of ordinary skillin the art would expect that at a high aqueous concentration and smalldisperse phase size, as described herein, disruption of micelles i.e.,emulsion disruption, would occur leading to agglomeration and particlegrowth. The present invention provides stable emulsions and constantmicelle size during reaction. Through control of the micelle size andstability, it follows that the methods of the present invention allowcontrol of the size, morphology and polydispersity of the resultingparticles.

[0033] One aspect of the invention provides a method for preparingparticles within reverse micelles having a disperse aqueous phase. Themethod involves forming an emulsion including a water content of betweenabout 1-40%, preferably between about 5-25%, and more preferably stillbetween about 10-15%. In another embodiment, the emulsion has a watercontent of at least about 20%, more preferably at least about 30%, morepreferably at least about 40%, and more preferably still at least about50%.

[0034] It is an advantage of the present invention that the particlesdescribed here have a continuous structure, “continuous structure”defined as comprising essentially one outer surface and aredistinguished from agglomerates of nanometer-sized particles. Many priorart methods produce such agglomerates from emulsions, the agglomeratescomprising a cluster of nanometer-sized particles resulting in severalouter surfaces. The present method yields particles free from suchagglomeration because a substantial portion of the micelles produces oneparticle per micelle. The resulting high surface areas are attributed toa plurality of inner surfaces within the particles which arise from amyriad of channels and pores penetrating the particles. The continuousstructure can either comprise a crystalline structure or an amorphousstructure.

[0035] The emulsion is a water in oil emulsion in which the oilfunctions as the continuous phase. Preferably the oil is a hydrocarbonwhere “hydrocarbon” is defined as having a formula C_(x)H_(y)O_(z), xand y being the same or different, both x and y are integers greaterthan or equal to one and z can be an integer greater than or equal tozero. In a preferred embodiment, the hydrocarbon can be any saturated orunsaturated carbonaceous species with a carbon number>5, substituted orunsubstituted, linear or branched. It can be aliphatic or aromatic.

[0036] A surfactant is included in the emulsion to stabilize the reversemicelles. The surfactant can be an ionic surfactant or a non-ionicsurfactant. A surfactant typically has a main hydrocarbon chain whereinone end of the chain is soluble in one of the immiscible liquids of theemulsion and the other end is soluble in the other immiscible liquid.Thus a structure of the micelle includes the disperse aqueous phasecontained by a surfactant “film” which isolates and stabilizes theaqueous phase from the hydrocarbon continuous phase. In one embodiment,the surfactant is an “ionic surfactant” in which the chain has a chargebalanced by a counterion. Preferred ionic surfactants include SDS, andsodium AOT. In another embodiment, the surfactant is a “non-ionicsurfactant” which possesses a neutral charge. Preferred non-ionicsurfactants include polyethoxylated alcohols, polyethoxylated phenols,oleates, etc.

[0037] The reverse micelles formed by methods of the present inventionhave a mean diameter in the nanometer range. Preferably, the reversemicelles have a mean diameter of less than about 20 nanometers,preferably less than about 15 nanometers, more preferable less thanabout 10 nanometers, and more preferable still less than about 5nanometers. The reverse micelles are stable in that they are not easilydisrupted in the course of metal oxide particle growth. Disruptionoccurs when the surfactant film is broken, causing the particle to enterthe continuous phase and possibly resulting in loss of dispersity anddesired morphology. Thus the preferred combination of hydrocarbon,surfactant and water content stabilizes reverse micelles that allow theformation of particles having desired attributes.

[0038] The particles are formed within the reverse micelles. Other priorart methods involve the formation of the particles outside of themicelle. It is believed that the method of the present inventionachieves control of desired particle size and morphology. The method isfacilitated by providing a surfactant system for the nanoemulsion thatallows for introduction of a reactant from the continuous phase to thedisperse phase without disruption of the disperse phase (e.g.,disruption of the micelles) even at the high aqueous content of theformulations of the invention. This can be achieved in accordance withthe teachings of the above-referenced application Ser. No. 08/739,509 ofYing while, in some cases, altering the surfactant system to increasethe overall surfactant concentration while optionally decreasing theratio overall of cosurfactant to surfactant. A simple screening test canbe used to determine a suitable surfactant system. The screening testinvolves providing an aqueous phase including reactant and/or carrier,if auxiliary carrier is used to transport the reactant from thecontinuous phase to the disperse phase, providing disperse phase fluidincluding the reactant or carrier concentration suitable for carryingout the reaction, and forming aqueous-in-nonaqueous emulsions anddetermining the stability of these emulsions.

[0039] The method allows particle formation within the reverse micellethrough chosen parameters such as methods of introducing a reactant tothe emulsion and particular reactants used. The reactant can be a fluidthat is miscible with the continuous phase of the nanoemulsion, or areactant carried in a fluid carrier that is miscible with the continuousphase. The reactant can react at the aqueous phase by reacting with theaqueous phase (e.g., reacting with water as in a hydrolysis reaction) orthe reactant can be a first reactant that reacts with a second reactantdissolved or suspended within the aqueous phase.

[0040] The particles formed by the method of the present invention canbe organic or inorganic. Inorganic particles include metals, metalalloys, metal oxides and mixed metal oxides, metal sulfides, metalnitrides, metal halides, metal arsenides, and the like. Organicparticles include polymers such as polysaccharides, phospholipides,polylipids, co-polymers, hydrogels, and the like.

[0041] In one embodiment, the invention provides a method for preparinga metal oxide particle. The metal can be selected from the groupconsisting of alkaline metals, alkaline earth metals, transition metalsand rare earth metals. Preferably, the metal oxide is an oxide of ametal from group IA, or IIA, such as magnesia, calcia, or baria, or fromgroup IIIA or an oxide of a transition metal such as titania, manganeseoxide, yttria, zirconia, lanthana, and the like. Also, oxides ofmetalloids or of semimetals are included as are oxides of lanthanidessuch as ceria, samaria, and the like. Also, oxides of actinides, andcombinations of the above, i.e., complex metal oxides, are included.Complex metal oxides can include perovskites such as La—Sr—Fe—Co oxide.Complex metal oxides also can include aluminates such as bariumhexaaluminate or strontium hexaaluminate. Also, titanates such strontiumtitinate, and also silicates. As mentioned, the metal oxide can be amixed metal oxide comprising at least two different metals.

[0042] Particles of the invention (material) can be defined by at leastone metal oxide doped with at least one metal oxide. Examples includedoped barium aluminates in which the dopant is a transition metal oxidesuch as Ni, Mn, La, Co, Fe, Cu, Y, or the like. Another example of adopant is a lanthanide or actinide such as Ce, Gd, Pr, or the like.

[0043] In another embodiment the particles of the invention (material)are at least one metal oxide supported on at least one different metaloxide. In one embodiment the support is barium hexaaluminate. Othersupports include titania, magnesia, and the like. The supported materialcan be, for example, cerium oxide, La/Ce oxides, Cu/Ce oxides, bariumoxide, nickel oxide, manganese oxide, cobalt oxide, vanadia, or thelike.

[0044] The material of the invention can also be supported on amonolith. A monolith, as would be known to those of ordinary skill inthe art, is a continuous solid having macropores of at least 1micrometer in size.

[0045] Prior art methods of synthesizing metal oxides in aqueoussolutions typically rely on a reaction rate of a metal reactant withwater. These methods presents difficulties in controlling particlemorphology especially when preparing mixed metal oxides or other complexmetal oxides because a first and second metal reactant may have verydifferent reaction rates with water. The method of the present inventionallows control of mixed metal oxide morphology because the various metalreactants can be dissolved within the continuous phase, the aqueousphase or the fluid carrier. Thus reaction rates are determined bydiffusion into the reverse micelle, and for metal reactants in general,these reaction rates are relatively equal.

[0046] In another embodiment, the metal reactant may react with aqueousphase after controlling the pH of the aqueous phase. Control of pH canbe accomplished by the addition of a suitable acid or base in desiredamounts. Suitable acids include: organic or inorganic acids. Inorganicacids include sulfuric acid, nitric acid, hydrochloric acid, phosphoricacid, carbonic acid, aqueous hydrogen bromide and iodide. Organic acidsinclude: acetic acid, any carboxylic acid.

[0047] Because conditions of pH, temperature, reactant concentration,and the like will be adjusted for a particular reaction that is to takeplace within the disperse phase of the nanoemulsion, in some cases, thesurfactant system should be tailored so as to preserve the emulsionunder these conditions. The above-described screening test can be usedfor tailoring the surfactant system for particular reaction conditions.

[0048] The rate of addition of reactant to the emulsion system should below enough that an unacceptable local over-concentration of reactantdoes not result. The rate should be low enough that the emulsion is notdisrupted. The control of the addition is done empirically. Theprecursor mixture can be contained in solution within a carrier or thecontinuous phase in an air-free flask (it can also be done in a normalatmosphere). The solution is then “pushed-out” through a cannula withargon, nitrogen or other gas, into the reverse emulsion which isvigorously stirred. Typical addition rates are in the order of 1 ml/minfor 100 ml emulsion volumes.

[0049] The carrier used to carry a reactant from the continuous phase tothe disperse phase, where a carrier is necessary, can be selected among,for example, for inorganic reactions alcohols with typically less thanabout 5 carbons of any configurations such as linear or branchedhydrocarbons (these include, for example, neopentanol, butanols,propanols, ethanol, etc.); for organic reactions: organic solvents suchas heptane, hexane, toluene, benzene, cyclohexane, etc.

[0050] In one embodiment the reactant is a ceramic precursor dissolvedin a carrier solvent that is miscible with the continuous phase of thenanoemulsion. The ceramic precursor can be miscible with the continuousphase of the nanoemulsion. In another embodiment, the ceramic precursoris introduced into the disperse phase of the reverse emulsion prior toreaction.

[0051] In another embodiment, a ceramic precursor in a non-aqueousemulsion is introduced into the aqueous reverse emulsion. The emulsifiedceramic precursor can be miscible with the continuous phase of thenanoemulsion. In yet another embodiment, the method involves introducinga ceramic precursor into the disperse phase prior to formation of theemulsion. In a preferred embodiment, the ceramic precursor defined is analkoxide precursor such as barium and aluminum alkoxide. Awater/precursor molar ratio can be about 1:500, preferably about 20:300and more preferably about 50:100. Accordingly, these embodiments can beused when the reactant is a base or an acid.

[0052] In another embodiment, the reactant ceramic precursor can beprecipitated by introducing a reactant which is miscible with thecontinuous phase of the reverse emulsion. The ceramic precursor can alsobe precipitated by introducing a reactant which is emulsified in asolvent which is miscible with the continuous phase of the reverseemulsion. The reactant can also be a base or an acid.

[0053] In one set of embodiments the reactant is a second emulsion. Forexample, first and second water-in-oil emulsions are provided in which afirst reactant is provided in the aqueous phase of the first emulsionand a second reactant, which reacts with the first reactant, is providedin the aqueous phase of the second emulsion. These can be first andsecond reactants dissolved or suspended in the respective aqueousphases. The emulsions are mixed and, since emulsions of these types aredynamic and fluid/fluid exchange occurs in a normal, dynamic mannerbetween the discontinuous, aqueous phases, the fluid/fluid exchangeleads to a reaction. As one example of this embodiment, one emulsion canbe provided in which the aqueous phase contains barium nitrate andaluminum nitrate, and the other emulsion contains an aqueous phase inwhich is dissolved ammonium hydroxide. Mixture of these two emulsionsresults in the production of barium and aluminum hydroxides, and oxides,with particle size and/or surface area as described above.

[0054] Reactions described in these embodiments can be promoted by anenergy source selected from the group consisting of a microwaveradiation source, a laser, an ultraviolet radiation source, an electricfield, a magnetic field, and an electromagnetic field.

[0055] Products generated via precipitation or chemical reaction in theaqueous phase of nanoemulsions having an aqueous concentration of atleast 5% can be used in suspension, or recovered. Recovery can involveproviding the particles reacted in the emulsion, and then freeze-dryingthe particles. The freeze-dried particles can then be supercriticallydried. Remaining surfactant and other organics can be removed bydisplacement with a fluid such as isoproponal and then supercriticallydrying the particles, or oven drying the particles. Alternatively, theparticles and auxiliary surfactant and other organics can be oven-driedor supercritically-dried, without freeze drying. The recovery method caninvolve inducing phase separation of the reverse emulsion by cooling orheating.

[0056] In another embodiment, the materials are recovered after an agingperiod, preferably between 1 h and 7 days. When the reaction includes abarium-aluminum reaction mixture a preferable aging time is betweenabout 1 h to 3 days before recovery, more preferably between about 12 to72 h, and more preferably still between about 24 to 48 h.

[0057] In another embodiment, the reacted particles in the nanoemulsioncan be supercritically dried by exchanging existing fluids with a fluidsuch as isoproponal, then supercritically drying the resultantsuspension.

[0058] Supercritical drying can involve exposing the system toconditions at which the fluid carrier is a supercritical fluid, andventing the supercritical fluid above its critical temperature.Freeze-drying can involve spraying the emulsion, through a nozzle oratomizer, into liquid nitrogen to create finely-divided frozen particlesof the emulsion, and then evacuating the particles at a temperature lowenough that sublimation of the carrier and other fluids occurs. Wherethe materials are a product of barium-aluminum precursor reactions,spray-freezing is a preferable method for particle recovery.

[0059] The method can further include removing remaining organicmaterials by a process selected from the group consisting of heattreatment, solvent extraction and rotoevaporation.

[0060]FIG. 1 shows a flowchart 10 describing the generalized synthesisprocedure. A water-in-oil reverse emulsion 12 is prepared by mixing asurfactant/cosurfactant combination, a hydrocarbon phase, and an aqueousphase. The order of addition does not affect the final state of theemulsion, although undesired metastable gel-like phases may be obtainedas intermediate products. To avoid the undesired product formation, thesurfactant is first dissolved in the hydrocarbon phase. The water andcosurfactant are then added alternatively. The resulting emulsion is acontinuous transparent liquid. This system is composed of monodispersewater droplets of nanometer size suspended in a continuous hydrocarbonphase, each droplet surrounded by a surfactant interface. Previousresearch describes a methodology to obtain reverse micellar solutionsusing a variety of hydrocarbon systems. The organic phase can be a purehydrocarbon, such as iso-octane, or a complex mixture, such as keroseneor diesel. The main surfactant is usually a complex mixture ofpolyethoxylated alcohols. The cosurfactant is either a linear alcoholsuch as hexanol or heptanol, or a mixture of linear alcohols andpolymeric surfactants such as polyethylene glycol alkylates. The finalparticle size of the water particles depends on the nature of thehydrocarbon used as well as on the characteristics of thesurfactant/cosurfactant system.

[0061] Once the water-in-oil emulsion is created, step 14 in FIG. 1involves dissolving the hydrolysis precursors in a suitable medium thatwill not cause phase separation of the emulsion. Examples of thesuitable medium are isopropanol, butanol or pentanol. The precursors forceramic applications are usually alkoxides of base metals or transitionmetals, such as iso-propoxides or sec-butoxides. The precursor solutionis then slowly added to the nanoemulsion. The precursors diffuse to themicelle interface where they come in contact with water, initiating thehydrolysis reaction. The diffusion rate of precursors to the hydrolyzingmedia can be controlled by manipulating the agitation rate, as well asthe steric configuration of the precursors. A transparent clear solutionis obtained after addition, indicating that the precipitated particlesobtained from the hydrolysis are in the nanometer range and do notscatter light. The inverted micellar compartments act as microreactorswhere the hydrolysis reaction is conducted. We believe that reactionproducts remain contained within the reversed micellar compartmentsthroughout the hydrolysis and condensation processes. In this way, theparticle size of the hydrolysis products, as well as their geometry, canbe controlled by manipulating the particle size and morphology of themicelles in the emulsion, and the aging conditions. Through aging, avariety of particle diameters can be obtained, ranging from tens toseveral hundred nanometers. The final particle size and morphology willdepend on parameters such as the agitation rate, the temperature duringgrowth, and the aging time. The products can be separated and treatedafterwards by a variety of techniques, depending on the desiredapplication.

[0062] According to another aspect, the invention provides a method ofmaking a high-surface-area and/or small-particle-size material.“Particle size” is defined as a mean diameter of a particle. The methodresults in particles of material, reacted at the aqueous phase, having aparticle size of less than about 100 nm in preferred embodiments, morepreferably less than about 50 nm, more preferably less than about 20 nm,more preferably less than about 10 nm, and more preferably still lessthan about 5 nm. The surface area of the material is preferably at leastabout 20 m²/g, more preferably at least about 50 m²/g, more preferablyat least about 75 m²/g, more preferably at least about 100 m²/g, morepreferably at least about 150 m²/g, and more preferably at least about200 m²/g. In another set of embodiments even higher surface areamaterials are produced, for example those of at least about 300, 400,500, 600, 700, 800, 900, or even 1000 m²/g. The material of theinvention is particularly robust and, as mentioned, is defined byhighly-divided, non-agglomerated, high surface area material. Thematerial preferably retains the surface area defined above even whenstored at room temperature for a considerable period of time or whenheated to higher temperatures. In particular, the material maintains theabove-noted surface areas when heated to 500° C., 700° C., 900° C.,1100° C. or even 1300° C. for periods of time for at least about 10minutes, preferably 30 minutes, more preferably 1 h, or more preferablystill at least about 2 h.

[0063] It is noted that one set of embodiments includes all combinationsof all of the above-noted surface areas, particle sizes, and retentionof surface area at the variety of temperatures mentioned.

[0064] According to one aspect the invention provides a series oflow-particle-size and/or high-surface-area particulate material.Preferably the particles have a size of less than about 100 nm, or otherpreferred average particle sizes described above, and/or have a surfacearea of at least about 20 m²/g, or any of other preferred surface areasdescribed above.

[0065] The material that is capable of synthesis or precipitation inaqueous conditions includes any inorganic or organic material capable ofundergoing a hydrolysis reaction in aqueous conditions, a precipitationreaction, a polymerization reaction, or the like. These reactions arewell-known to those of ordinary skill in the art. For example,polymerization can be effected by chemical initiation or by, forexample, exposure to electromagnetic radiation such as UV light.Hydrolysis can be followed by polymerization, polycondensation,coupling, or the like. Based upon the teachings in the instantdisclosure, those of ordinary skill in the art can select a wide rangeof materials capable of synthesis or precipitation in aqueousconditions.

[0066] In one set of embodiments, the material that is capable ofsynthesis or precipitation in aqueous condition is a material capable ofparticipating in a catalytic combustion reaction, e.g., for ultra-leancombustion. These types of materials, at high combustion temperatures,can operate at low fuel concentration and can reduce emissions ofpollutants such as NO_(x). But at temperatures high enough for this typeof combustion, typical prior art materials undergo unacceptablevaporization, sintering, or other degradation. In one embodiment, thematerial of the present invention that is capable of synthesis orprecipitation in aqueous conditions includes Group IIA oxides orhexaluminates (such as BaO.6Al₂O₃) optionally including dopantstypically used in catalytic combustion reactions such as transitionmetals, metalloids, complexes of these materials, transition metaloxides, actinides, lanthanides, oxides of these, Group IA elements, orcombinations thereof.

[0067] In one set of preferred embodiments the material of the inventionis stable at a temperature of at least 500° C. for at least 2 hours withloss in surface area of the material of less than about 10%. In anotherset of preferred embodiments the material of the invention is stable,after being heated at a temperature of at least 500° C. for at least 30minutes, for at least two additional hours with a loss in surface areaof the material of less than about 10% during that two hours. In a setof more preferred embodiments this maximum loss in surface area occurswhen the material is heated at a temperature of at least about 750° C.,preferably at least about 1000° C., more preferably at least about 1100°C., more preferably at least about 1300° C., and more preferably stillat least about 1500° C. for a period of time for at least about 2 hours,preferably at least about 8 hours, more preferably at least about 12hours, and more preferably still at least about 24 hours. Anycombination of these parameters can define one embodiment. For example,in one embodiment the material is stable at a temperature of at leastabout 1300° C. for a period of time of at least about 8 hours with aloss in surface area of less than about 10%.

[0068] This material can be any of the above-described materials such asmetal oxides, Group IIA hexaluminates or oxides, any combinations ofthese materials optionally doped, and the like.

[0069] The particles produced in accordance with the invention have avariety of uses. In one embodiment the particles are used asdrug-delivery carriers since very small organic particles (e.g.,polymeric particles) can be created encapsulating a therapeutic agent.For example, a therapeutic agent can be provided in the aqueous phase ofthe nanoemulsion and a reactant introduced into the continuous phase,optionally via a carrier miscible with the continuous phase, and allowedto react at the aqueous/nonaqueous interface to form an organic(optionally polymeric) shell encapsulating the therapeutic agent. Thesenano-sized particles can be used as drug-delivery agents.

[0070] In another set of embodiments, nano-scale particles can beprovided for use in micro or nanochromatography. For example, very smallsolid-phase chromatography beads can be prepared in accordance with theteachings of the invention, a suspension of the beads can be flowedthrough a capillary tube, and carrier can be driven off by heat.

[0071] The invention also provides a technique for controlling thelength of polymer chains in a polymer reaction by confining the spacewithin which the reaction can occur. The confined space is defined bythe microreactors that are the aqueous micelles of the nanoemulsion.When a polymeric precursor is provided in the aqueous phase and areactant added to the discontinuous phase which is allowed to interactwithin the aqueous phase and cause polymerization, because of the sizeof the aqueous micelles, polymer chain size is limited. Additionally,nanometer-sized polymer particles can be made.

[0072] Another aspect of the invention provides a method for oxidizinghydrocarbons. The oxidation is performed in the presence of one or morenon-noble metal oxides with particle size of less than about 100 nm,preferably less than about 50 nm, more preferably less than about 25 nm,more preferably less than about 10 nm, and more preferably still lessthan about 5 nm. The hydrocarbon can be selected from the groupconsisting of methane, ethane, propane and butane. A preferableoxidation temperature is between about 400° C. to about 1300° C.

[0073] The invention provides a hydrocarbon conversion of at least about10% at about 400° C., preferably at least about 10% at about 350° C.,more preferably at least about 90% at about 600° C., and more preferablystill at least about 90% at about 500° C. In another embodiment, atleast about 90% of the initial catalytic conversion is sustained at1100° C. for at least about 2 h, preferably for at least about 12 h, andmore preferably for at least about 1 month. In another embodiment, atleast about 90% of the initial catalytic conversion is sustained at1300° C. for at least about 2 h, preferably for at least about 12 h, andmore preferably for at least about 1 month.

[0074] In another embodiment the hydrocarbon catalysis occurs in thepresence of water or water vapor. Preferably the catalysis occurs at1300 ° C. in 8% volume water.

[0075] In all aspects of the invention, the particles can be any of thedescribed preferred particles in any of the described particle size orsurface area ranges or combinations of particle size and surface area.The methods of making compounds can be used, and preferably are used, incombination with preferred sizes and surface areas of the particles asdescribed herein.

[0076] The function and advantage of these and other embodiments of thepresent invention will be more fully understood from the examples below.The following examples are intended to illustrate the benefits of thepresent invention, but do not exemplify the full scope of the invention.

EXAMPLE 1 Preparation of the Reverse Nano-emulsion

[0077] Water-in-oil nanoemulsions were prepared by mixing 2,2,4trimethylpentane (Aldrich Chemical Co. Inc., 99.8+% pure) and de-ionizedwater with a commercial polyethoxylated alcohol surfactant, Neodol 91-6(Shell Chemical Co.), as main surfactant, and 1-pentanol (AldrichChemical Co. Inc., 99+% pure) as cosurfactant. The emulsions werestirred vigorously until a clear solution was obtained. Emulsionscontaining 1, 5, 10, 30 and 40 wt % water were prepared. Table 1 showsthe amounts of surfactant and cosurfactant required to prepare theseemulsions.

[0078] Addition of acetic acid (Aldrich Chemical Co. Inc., 99.8% pure)or ammonium hydroxide (Aldrich Chemical Co. Inc., 99.9% pure, 30% inwater) to the water used to form the emulsion, was used to control thepH of the hydrolyzing media. The acid or hydroxide was dissolved in thedeionized water prior to the mixing with the organic and surfactantphases. Emulsions containing water phases of pH 2, 4, 7, 10, and 12 wereused for these experiments. Table 1 shows the variation of the overallamount of surfactant required to stabilize microemulsions with varyingthe pH of the aqueous phase.

[0079] Introduction of dopants into the barium hexaaluminate structurecan be desirable in order to improve its catalytic combustionproperties. Transition metal dopants such as manganese and nickel havebeen proven to increase the catalytic activity of barium hexaaluminateat low temperatures. In order to effectively increase the catalyticactivity, these cations are incorporated into the lattice structure,substituting an aluminum atom in the crystal. Nickel salts, such asnickel nitrate can be dissolved in the water used to prepare theemulsions, in an attempt to obtain a doped barium hexaaluminate throughthis method

EXAMPLE 2 Preparation of the Mixed Ba—Al Alkoxide Solution

[0080] 2-Propanol (Malinckrodt Chem. Co., 99+% pure) was degassed anddried by argon (BOC Gases, Grade 5.0) exchange. 1 g of metallic barium(Aldrich Chem. Co. Inc., 99+% pure) was mechanically milled and placedin a 1000 ml dried flask. As an alternative to mechanical milling, thebarium can be reacted as a pellet, ingot, wire or the like. 250 ml ofdried 2-propanol was transferred to the flask under argon and refluxedat 110° C. for 24 h. 9.6 g of aluminum isopropoxide (Aldrich Chem. Co.Inc., 98+% pure) were added to the barium isopropoxide solution andrefluxed for 24 additional hours. The combined barium/aluminum precursorconcentration was 5.42 wt %. No solid residues were observed afterrefluxing. Some of the resulting precursors solutions were furtherdissolved in dried 2-propanol in 1:2 and 1:4 ratios, yielding finalprecursors concentrations of 2.71 wt % and 1.35 wt % respectively.

[0081] We found that concentration of the precursors can affect the waythe reaction is carried out. When the precursor is introduced at mediumand high concentrations, the microemulsions typically will undergo phaseseparation, regardless of the water concentration and reactiontemperature. At the high concentrations, the resulting gels may be nodifferent than those prepared through regular sol-gel processing interms of surface area and thermal stability. At medium concentrations anincrease of the surface area of the dried gels is observed over regularsol-gel materials. However, after calcination at 1350° C., the surfaceareas are practically identical for both systems. Materials prepared atlow precursor concentration yield the highest surface areas for both thedried gels and calcined materials.

[0082] At low precursor concentrations, the emulsion is maintained, andthroughout the hydrolysis process, the solution remains clear. TEManalysis of the reaction mixture prepared using dilute precursorsolutions revealed a uniform particle size distribution and virtually noagglomeration. FIG. 2 shows a TEM of the microspheres prepared from a 5wt % water emulsion using dilute precursor solution. At low precursorconcentration hydrolysis and condensation are limited to the waterdroplets in the microemulsion. This regulates the growth of thenanoparticles uniformly within the nanometer-sized inverted micellarcompartment.

[0083] As-prepared gels were recovered by filtration. XRD analysis ofthese gels gave the diffraction pattern of an amorphous material. Afterdrying at 120° C. for 24 h, nitrogen adsorption studies of these gelswere performed. A typical isotherm is shown in FIG. 3(a) and thedesorption pore size distribution is shown in FIG. 3(b). In general, thesamples showed an adsorption/desorption isotherm characteristic of amesoporous material at 77 K. The surface area of the preparations rangedfrom 200 to 650 m²/g, while the pore diameter was in all cases uniformand around 4-10 nm. A significant hysteresis was observed between theadsorption and desorption isotherms. We attribute this behavior tointer-particle porosity, deriving from particle arrangement. Thisporosity may involve small pore openings with a larger void fraction inbetween particles, effectively creating an “ink-bottle” type pore. SEMstudies of these dried gels showed spherical particles of uniform sizeas shown in FIG. 4.

EXAMPLE 3 Hydrolysis and Aging of the Alkoxide Precursors

[0084] The alkoxide solutions were added to the nanoemulsions atdifferent rates, from 0.1 to 10 ml/min, and under various temperatures,from 25 to 80° C. The resulting gels were aged for different timelengths at a range of temperature and agitation conditions.

[0085] Effect of Water Content and pH in the Hydrolyzing Emulsion. Thewater content was found to affect the degree of agglomeration and theyield of materials per gram of emulsion. The high and medium waterconcentrations were found to give less agglomeration of the hydrolyzedmaterials and a more uniform particle size than the low-end values. FIG.5 shows a plot of the particle size distribution as observed in TEM fordifferent water contents. After aging for 8 h at room temperature, lowwater concentrations, below 5 wt %, were observed to yield agglomeratedparticles of 100-150 nm in diameter. When the water content in theemulsion was raised to 5 to 20 wt %, particles of 50-100 nm wereobtained. Optimum water content was found to be between 5 and about 20wt %. Water content higher than 25% sometimes yielded additionalcrystalline phases with a decrease in final surface area. While ingeneral, optimum water content was between about 5 and 20 wt %, in somecases, for water contents from 30 wt % to 40 wt %, virtually noagglomeration was observed, while the particles obtained ranged in sizefrom 10 to 50 nm. The average particle size for each preparation wasobtained by measuring particle diameters in the transmission electronmicrographs of the aged solutions.

[0086] DLS measurements of the initial emulsions showed that for allwater contents the micellar size was below 5 nm and was monodisperse.Furthermore, we observed little variation of the micelle diameter withchanging water content. However, the intensity of the signal of theparticle size interval<5 nm increased with increasing water content,indicating that additional water added to the system forms new micellesrather than increasing the size of existing bodies. These results areconsistent with the observations describing particle formation as afunction of water concentration in the emulsion: at higher water contenta larger number of micelles are available as hydrolysis nuclei, hencethe nucleation rate is increased, leading to uniform, monodispersedspheres.

[0087] The values for the yield of final product as a function of watercontent were also investigated. We used emulsions with water contents of1, 5, and 30 wt % corresponding to twice the stoichiometric valuerequired to hydrolyze a solution containing 2.71 wt % of barium andaluminum isopropoxide in isopropanol. The results show a 100% conversionof the barium hexaaluminate precursors for the 5 and 30 wt % watercontent emulsions with yields of 0.11 and 0.68 g of barium hexaaluminateper 100 g of emulsion respectively, after aging for 8 h at roomtemperature. The 1 wt % water emulsion resulted in a conversion of only67% under the same conditions, and the final yield was 0.015 g per 100 gof emulsion. Regular sol-gel processing yields 2.26 g of bariumhexaaluminate per 100 g of water under the same stoichiometricconditions.

[0088] The effect of pH of the hydrolyzing media was also investigated.At pH of 7 and below, virtually no agglomeration was observed. At pH of8 and above, immediate macroscopic particle formation and phaseseparation are observed after the precursor solution is added.

[0089] Effect of Aging Conditions. The best materials were obtained whenthe hydrolysis and aging processes were conducted at the lowertemperature value. At temperatures above 40° C. significant particlegrowth was evident from macroscopic precipitation of large rod-shapedparticles. When observed through SEM these particles showed evidence ofheavy agglomeration of smaller spherical entities. The surface areaafter drying of the materials synthesized at 40 and 80° C. was nodifferent from those prepared by regular sol-gel techniques.

[0090] Aging time was found to influence the final particle sizeobtained after recovery. We observed that particle size increased almostlinearly with time for the first 24 h. The optimal aging time wasdetermined to be about 24 h. After aging for periods longer than 24 h,the particles start becoming visible, as indicated by a change of thetransparency of the emulsion. If stirring is stopped, a very finedust-like suspension is observed. Aging emulsions were able to be keptin one phase for water contents as low as about 15 wt %. Aging times of8 h or less produced materials with the highest surface area and lowestdegree of agglomeration.

[0091] Through aging for extended periods of time, of up to 96 h,particles of uniform diameters in the micrometer range can be obtained.Samples prepared using low precursor concentrations, hydrolyzingemulsions containing 30 wt % water, and aged at room temperature for 72and 96 hours yielded uniform spheres of 1.66 and 6.67 μm averagediameter. These materials showed surface areas as high as 200 m²/g afterdrying. FIG. 6 shows an optical micrograph of microspheres aged for 72h.

[0092] In summary, we found that the following processing conditionsgive the best materials in terms of final surface area, thermalstability, degree of agglomeration, and particle size distribution: lowprecursor concentration of 1.35 wt %, water content of about 15-20 wt %in the hydrolyzing emulsions, water-to-precursor ratio of about 15 molarto about 100 molar, short aging time of 12-24 h and low agingtemperature of 25° C.

EXAMPLE 4 Particle Recovery

[0093] The hydrolyzed and aged gels were recovered using two methods. Inthe first procedure, 2-propanol was added until a macroscopic two-phaseseparation of the emulsion was observed. The lighter phase contained thehydrocarbon organic phase, the alcohol cosurfactant, and a small amountof the main surfactant and 2-propanol; while the heavier phase containedthe hydrolyzed gel, the remaining surfactant, water and 2-propanol. Thelighter phase was decanted out. The heavier phase was dried and calcinedaccording to section 2.2.5.

[0094] The second recovery method involved freezing the whole systemrapidly by spraying it into a liquid nitrogen-cooled flask. Thesolidified solution was scraped from the flask and placed in a VirtisCo. Cascade Freeze Drier. The organic phase, water, cosurfactant and2-propanol were selectively sublimed, leaving behind a gel consisting ofmainly the surfactant and the hydrolyzed products, with a small contentof water and 2-propanol. This gel was then treated according to Example5.

EXAMPLE 5 Drying and Sintering

[0095] The recovered gels were re-suspended in dried 2-propanol. In oneprocedure, the mixture was refluxed in 2-propanol at 80° C. for 8 to 24h. In a second approach, the gels were placed in a stirred beakercontaining dried propanol and ultra-sonicated for a total of 2 h. Somepreparations were centrifuged and re-suspended in fresh 2-propanol for atotal of 4 times, for the purpose of eliminating most of the surfactantbefore drying. After re-suspension and washing, the particles wereeither oven dried at 120° C. for 24 h, supercritically-dried usingnitrogen as the purging fluid, or freeze-dried. Remaining surfactant wasremoved by subsequently heating under nitrogen to 400° C. for 2 h andoxygen at 400° C. for 4 h. After surfactant removal, most systems wereexamined for high temperature stability by heating under oxygen at 5°C./min to 1300° C. and/or 1500° C. and held for 2 h to 8 h.

[0096] Effect of Recovery and Drying Techniques. Samples prepared usingthe optimal values obtained from the partial factorial experimentaldesign were treated with different recovery and drying techniques.Samples treated by freeze drying, followed by re-suspension in2-propanol and supercritical drying showed extraordinarily large surfacearea and narrow pore size distribution after calcination to temperaturesas high as 1350° C. Pure barium hexaaluminate phases with surface areasexceeding 90 m²/g were obtained using this technique. The average porediameter of these samples was 40 nm. We believe that the dryingprocesses are critical in preserving the particle and pore integrity. Byusing freeze drying we are able to remove the organic and polar solventsthrough sublimation, leaving the heavier surfactant behind. Thesurfactant is then exchanged using short-chain alcohols, which arefurther removed through supercritical drying. This last step preventshigh capillary pressure within the particles during evaporation andhighly exothermic reactions such as burning of the surfactants,preserving the particle and pore structure. Table 3 gives a summary ofthe observed surface area and crystalline phases present in differentmaterials. Spherical particles are observed at temperatures as high as500° C., after which sintering occurs giving way to agglomeratedcrystalline structures.

[0097] Recovery of particles through centrifugation was also evaluated.This technique was found to lead to agglomeration and disruption of thespherical particles, resulting in low surface areas, comparable to thoseobtained through regular sol-gel processing.

EXAMPLE 6 Characterization

[0098] X-ray diffraction (XRD) patterns of the samples after drying,surfactant removal, and calcination were taken on a Siemens D5000 θ-θdiffractometer with nickel-filtered Cu Kα radiation (1.5406 Å). Surfaceareas and average pore diameter of samples were determined in a nitrogenadsorption apparatus (Micromeritics ASAP 2000) using BET and BJHdesorption methods respectively. Transmission electron microscopy (TEM)was performed on a JEOL 200 CX microscope operating at 200 kV. Surfacemorphology of the samples was observed through field-emission scanningelectron microscopy using a JEOL 6840 FESEM.

[0099] The hydrolyzing emulsions were characterized for micelle size andmorphology through dynamic laser light scattering (DLS) using aBrookhaven AT-9000 Correlator.

EXAMPLE 7 Control Particle Using a Sol-Gel Process

[0100] A control sample of barium hexaaluminate was synthesized througha conventional sol-gel process as described by Machida et al. (M.Machida, K. Eguchi, and H. Arai, Chem. Lett. (1987) p. 267) forcomparison purposes. A surface area of 80 m²/g was obtained after dryingto 500° C., which dropped to 16 m²/g after calcination at 1300° C. for 2h. XRD analysis of this sample showed a pure barium hexaaluminate phasepresent after calcination.

EXAMPLE 8 Experimental Design

[0101] A partial factorial experimental design was performed to evaluatethe relevant variables and their influence in the properties of theresulting materials. Table 2 shows the ranges through which theprocessing parameters were varied. These ranges were selected accordingto preliminary results obtained from regular sol-gel processing. Theevaluated properties were surface area of the dried powders, and surfacearea and XRD phases after calcination to 1350 ad 1500° C.

EXAMPLE 9 Synthesis of Doped Barium Hexaaluminate

[0102] Prior to the preparation of the emulsion, 0.125 g of Cerium (III)Nitrate (Aldrich, 99%) were dissolved into 26.85 g of deionized water.The cerium nitrate solution was emulsified as described before. Thefinal composition was: Cerium nitrate 0.125 g Deionized water 26.85 gIso-octane 152 g Neodol 91-6 52 g 1-Pentanol 28 g

[0103] 20 ml of a solution of barium and aluminum isopropoxidecontaining 0.7 g of Ba per 100 ml of solution and 10.8 g of Alisopropoxide per 100 ml of solution, was further diluted with 20 ml ofisooctane. The final solution was added at a rate of 1 ml/min to theemulsion described above. The reaction mixture was aged for 24 h at roomtemperature.

[0104] The resulting materials were recovered by freeze drying andcalcined under air to 500 and 800° C. Analysis through XRD showed thecerium crystallized into cerium (IV) oxide with grain sizes of less than5 nm at 500° C. and less than 7 nm at 800° C. The surface area of thepreparation was 455 m²/g at 500° C. and 220 m²/g at 800° C.

[0105] The method described for cerium can also be used for Ni, La, Co,Fe, Mn.

EXAMPLE 10 Synthesis of Coated Barium Hexaaluminate

[0106] A reverse emulsion was synthesized as described in Example 1. Thefinal composition of the emulsion was: Water: 26.825 g Iso-octane: 152 gNeodol 91-6: 52 g 1 Pentanol: 28 g

[0107] 30 ml of a solution of barium and aluminum isopropoxidecontaining 0.7 g of Ba per 100 ml of solution and 10.8 g of Alisopropoxide per 100 ml of solution, was further diluted with 30 ml ofisooctane. The final solution was added at a rate of 1 ml/min to theemulsion described above. The reaction mixture was aged for 18 h at roomtemperature.

[0108] After aging, 0.125 g of cerium (III) nitrate were added to theemulsion containing the hydrolyzed barium and aluminum precursors. Themixture was further aged for 6 h.

[0109] The materials were recovered by freeze drying and calcined underair to 500, 800 and 1100° C. After calcination, the materials wereanalyzed by XRD, nitrogen adsorption, and high-resolution transmissionelectron microscopy (HRTEM). XRD showed cerium crystals developed with asize of less than 4 nm at 500° C., less than 6.5 nm at 800° C., and lessthan 16 nm at 1100° C. No barium hexaaluminate phases were detected.Nitrogen adsorption showed a surface area of 409 m²/g at 500° C., 299m²/g at 800° C., and 109 m²/g at 1100° C. HRTEM showed the bariumhexaaluminate particles were coated with highly dispersed, uniformcerium oxide crystallites, which retained nanocrystalline morphologiesbeyond 1100° C. A microstructure of Ce—BHA is shown in FIG. 7.

EXAMPLE 11 Catalytic Oxidation of Methane with Pure BHA Materials

[0110] A tubular, plug-flow reactor was used to determine the catalyticactivity of BHA materials. 0.87 g of pure BHA having a surface area of112 m²/g was introduced into a ¼″ tubular quartz reactor. A mixture of1% CH₄ in air was flowed at a space velocity of 60,000 h⁻¹ over thecatalyst. The reactor was kept at constant temperature using a furnace.The conversion of CH₄ was measured by analyzing the exhaust stream usinga gas chromatograph coupled to a mass spectrometer (GC/MS). 10% methaneconversion was observed at 600° C., full conversion was achieved at 750°C. A plot of temperature versus methane conversion is shown in FIG. 8.

EXAMPLE 12 Catalytic Oxidation of Methane with Doped BHA Materials

[0111] Manganese doped-BHA was prepared as described in example 9 with atotal loading of Manganese of 3 wt % by elemental analysis. The samplehad a surface area of 75 m²/g after calcination to 1300° C. XRD analysisshowed Mn₃O₄ diffraction peaks, in addition to the BHA peaks. The sameprocedure described in example 11 was used to probe the catalyticactivity of the material. 10% conversion of methane was achieved at 520°C., full conversion was obtained by 650° C.

EXAMPLE 13 Catalytic Oxidation of Methane with Coated BHA Materials

[0112] Cerium-coated BHA was prepared as described in example 10 with atotal loading of cerium of 10 wt % by elemental analysis. The sample hada surface area of 239 m²/g after calcination to 800° C. XRD analysisshowed CeO₂ diffraction peaks only. The same procedure described inexample 11 was used to probe the catalytic activity of the material. 10%conversion of methane was achieved at 430° C., full conversion wasobtained by 600° C. A plot of temperature versus methane conversion isshown in FIG. 9. TABLE 1 Formulation of microemulsions at differentwater contents. Trimethyl- Co- pentane Surfactant surfactant pH of theWater content content content content water phase (wt %) (wt %) (wt %)(wt %) 7 1 96 2 1 7 5 89 4 2 7 10 75 11 4 7 30 47 17 6 7 40 42 13 5 2 587 6 2 4 5 87 6 2 10 5 85 7 3 12 5 85 7 3

[0113] TABLE 2 Partial factorial experimental design: investigatedranges for processing variables. Ranges Variable Low Medium HighPrecursor 1.35 wt % 2.71 wt % 5.42 wt % Concentration Water in emulsion1 wt % 5 wt % 30 wt % Hydrolysis 25° C. 40° C. 80° C. TemperatureAddition Rate 0.1 ml/min 1 ml/min 10 ml/min Aging Temperature 25° C. 40°C. 80° C. Aging Time 8 h 24 h 48 h

[0114] TABLE 3 Summary of materials obtained through differentprocessing techniques Particle Surface area (m²/g) after: BaO.6 Water inRecovery Drying Drying Drying Calcination Al₂O₃ XRD Emulsion MethodMethod to 120° C. to 500° C. to 1350° C. Phases 5% PrecipitationConventional 200 Pending 15 Pure 5% Freeze Conventional 150 Pending 25Pure 1% Precipitation Conventional  50 Pending  8 Pure 5% PrecipitationSupercritical N/A 350 56 Pure 1% Precipatation Supercritical N/A 288 47Pure 5% Freeze Supercritical N/A 387 94 Pure 30% PrecipatationConventional 298 215 38 Mixed Al₂O₃ + BaO 30% PrecipitationSupercritical N/A N/A 87 Pure 30% Freeze Supercritical N/A 287 PendingPure

[0115] Those skilled in the art would readily appreciate that allparameters listed herein are meant to be exemplary and that actualparameters will depend upon the specific application for which themethods and apparatus of the present invention are used. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, the invention may be practiced otherwise thanas specifically described.

What is claimed is:
 1. A composition comprising a material having an average particle size of less than about 100 nm wherein the material, when heated to 700° C., retains an average surface area of at least about 100 m²/g.
 2. A composition as in claim 1, the material having an average particle size of less than about 50 nm wherein the material, when heated to 700° C., retains an average surface area of at least about 150 m²/g.
 3. A composition as in claim 1, the material having an average particle size of less than about 25 nm wherein the material, when heated to 700° C., retains an average surface area of at least about 200 m²/g.
 4. A composition as in claim 1, the material having an average particle size of less than about 10 nm wherein the material, when heated to 700° C., retains an average surface area of at least about 300 m²/g.
 5. A composition as in claim 1, the material having an average particle size of less than about 5 nm wherein the material, when heated to 700° C., retains an average surface area of at least about 400 m²/g.
 6. A composition as in claim 1, wherein the material is a ceramic material.
 7. A composition as in claim 1, wherein the material is a metal oxide selected from the group consisting of Group IA metal oxides, Group IIA metal oxides, Group IIIA metal oxides, transition metal oxides, an oxide of a metalloid, an oxide of a semimetal, an oxide of a lanthanide, an oxide of an actinide and combinations thereof.
 8. A composition as in claim 7, wherein the metal oxide is selected from the group consisting of magnesia, calcia, baria, titania, manganese oxide, yttria, zirconia, lanthana, ceria, samaria and combinations thereof.
 9. A composition as in claim 7, wherein the oxide is a complex metal oxide having at least two metal types.
 10. A composition as in claim 9, wherein the complex metal oxide is selected from the group consisting of a perovskite, an aluminate, titanate, silicate and combinations thereof.
 11. A composition as in claim 10, wherein the complex metal oxide is selected from the group consisting of La—Sr—Fe—Co oxide, barium hexaaluminate, strontium hexaaluminate and strontium titanate.
 12. A composition as in claim 1, wherein the material comprises at least one metal oxide doped with at least one metal oxide.
 13. A composition as in claim 1, wherein the material comprises at least one metal oxide supported on at least one metal oxide.
 14. A composition as in claim 1, wherein the material is immobilized on a surface of a monolith.
 15. A composition as in claim 1, wherein the material retains an average surface area of at least about 300 m²/g at room temperature.
 16. A composition as in claim 1, wherein the material, when heated to at least 500° C., retains an average surface area of at least about 100 m²/g.
 17. A composition as in claim 1, wherein the material, when heated to at least 900° C., retains an average surface area of at least about 100 m²/g.
 18. A composition as in claim 1, wherein the material, when heated to at least 1100° C., retains an average surface area of at least about 20 m²/g.
 19. A composition as in claim 1, wherein the material, when heated to at least 1300° C., retains an average surface area of at least about 20 m²/g.
 20. A composition comprising: a material capable of catalyzing a combustion reaction of a hydrocarbon, the material having an average surface area, after exposure to conditions of at least about 1300° C. for at least about 2 hours, of at least 20 m²/g.
 21. A method comprising effecting a reaction via introducing a water-reactive reactant in the presence of a reverse emulsion and recovering a material from the reaction having an average particle size of less than about 100 nm wherein the material, upon exposure to 700° C. for at least about 10 mins., retains a surface area of at least about 100 m²/g.
 22. A method as in claim 21, wherein the step of introducing the reactant comprises introducing a ceramic precursor into the reverse emulsion prior to reaction.
 23. A method as in claim 54, wherein the step of introducing the ceramic precursor into the reverse emulsion comprises dissolving the ceramic precursor in a solvent that is miscible with a continuous phase of the emulsion.
 24. A method as in claim 54, wherein the ceramic precursor is miscible with a continuous phase of the emulsion.
 25. A method as in claim 54, comprising introducing the ceramic precursor into a discontinuous phase of the emulsion prior to reaction.
 26. A method as in claim 21, comprising introducing a ceramic precursor in a non-aqueous emulsion into the reverse emulsion.
 27. A method as in claim 54, comprising introducing a ceramic precursor into an aqueous phase of the reverse emulsion prior to formation of the emulsion.
 28. A method as in claim 54, comprising effecting the reaction by applying energy from an energy source to reactants in the presence of the reverse emulsion.
 29. A method as in claim 28, wherein the energy source is selected from a group consisting of a microwave radiation source, a laser, an ultraviolet radiation source, and an electric, magnetic, or electromagnetic field.
 30. A method as in claim 54, comprising recovering particulate product by inducing phase separation of the reverse emulsion by a process selected from the group consisting of cooling and heating.
 31. A method as in claim 54, comprising recovering particulate product by spray-freezing the reverse emulsion.
 32. A method as in claim 54, comprising recovering particulate product after aging.
 33. A method as in claim 22, wherein the ceramic precursor comprises an alkoxide.
 34. A method as in claim 54, wherein the reverse emulsion contains from about 1 to about 40 wt % water.
 35. A method for preparing a particle, comprising: providing an emulsion including a hydrocarbon, at least one surfactant and a water content of about 1-40% to form reverse micelles, the reverse micelles comprising a disperse aqueous phase; adding at least one water-reactive reactant; and allowing the at least one reactant to react in and with the disperse aqueous phase to form a particle having a particle size of less than about 100 nm, the particle being free from agglomeration.
 36. A method as in claim 59, wherein the surfactant is a non-ionic surfactant.
 37. A method as in claim 59, further comprising adding a base prior to reaction in and with the disperse aqueous phase.
 38. A method as in claim 59, wherein the particle has an average surface area of at least 20 m²/g.
 39. A method as in claim 35, wherein the particle is a metal oxide particle.
 40. A method as in claim 39, wherein the metal oxide particle is a mixed metal oxide particle comprising at least two metals.
 41. A method for preparing a particle, comprising: providing an emulsion including a hydrocarbon, at least one non-ionic surfactant and a water content of about 1-40% to form reverse micelles, the reverse micelles comprising a disperse aqueous phase; adding at least one reactant; and forming a particle having a particle size of less than about 100 nm, the particle being free from agglomeration.
 42. A method comprising coating a particle within a micelle.
 43. A method as in claim 42, wherein the particle is coated with a metal oxide layer.
 44. A method as in claim 42, wherein the micelle is a reverse micelle.
 45. A method as in claim 42, wherein the particle is a metal oxide particle.
 46. A method comprising: providing a composition having a surface area of at least 20 m²/g after exposure to conditions of at least 1300° C. for at least 2 h; and oxidizing a hydrocarbon.
 47. A method as in claim 46, wherein the composition comprises particles having a particle size of less than about 100 nm.
 48. A method as in claim 46, wherein the hydrocarbon is selected from the group consisting of methane, ethane, propane and butane.
 49. A method as in claim 46, wherein conversion of the hydrocarbon is at least 10% at 400° C.
 50. A method as in claim 49, wherein at least 90% of the conversion is sustained at 1100° C. for at least 2 h.
 51. A method comprising oxidizing at least one hydrocarbon in the presence of at least one non-noble metal oxide having a particle size of less than about 100 nm.
 52. A method as in claim 51, wherein the metal oxide has a surface area of at least about 20 m²/g
 53. A method as in claim 33, wherein the alkoxide comprises barium alkoxide.
 54. A method comprising: introducing a water-reactive reactant comprising a ceramic precursor including barium alkoxide and aluminum alkoxide in the presence of a reverse emulsion; effecting a reaction; and recovering a material from the reaction having an average particle size of less than about 100 nm wherein the material, upon exposure to 700° C. for at least about 10 min., retains a surface area of at least 100 m²/g.
 55. A method as in claim 54, wherein the recovered material comprises barium hexaaluminate.
 56. A method as in claim 35, wherein the water-reactive reactant comprises a ceramic precursor.
 57. A method as in claim 56, wherein the ceramic precursor comprises an alkoxide.
 58. A method as in claim 57, wherein the alkoxide comprises barium alkoxide.
 59. A method for preparing a particle, comprising: providing an emulsion including a hydrocarbon, at least one surfactant and a water content of about 1-40% to form reverse micelles, the reverse micelles comprising a disperse aqueous phase; adding water-reactive reactants comprising barium alkoxide and aluminum alkoxide; and allowing the water-reactive reactants to react in and with the disperse aqueous phase to form a particle having a particle size of less than about 100 nm, the particle being free from agglomeration.
 60. A method as in claim 59, wherein the particle comprises barium hexaaluminate.
 61. A method as in claim 54, wherein the recovering step comprises recovering a material from the reaction having an average particle size of less than about 50 nm wherein the material, upon exposure to 700° C. for at least about 10 min., retains a surface area of at least about 150 m²/g.
 62. A method as in claim 54, wherein the recovering step comprises recovering a material from the reaction having an average particle size of less than about 25 nm wherein the material, upon exposure to 700° C. for at least about 10 min., retains a surface area of at least about 200 m²/g.
 63. A method as in claim 54, wherein the recovering step comprises recovering a material from the reaction having an average particle size of less than about 10 nm wherein the material, upon exposure to 700° C. for at least about 10 min., retains a surface area of at least about 300 m²/g.
 64. A method as in claim 54, wherein the recovering step comprises recovering a material from the reaction having an average particle size of less than about 5 nm wherein the material, upon exposure to 700° C. for at least about 10 min., retains a surface area of at least about 400 m²/g.
 65. A method as in claim 54, wherein the recovering step comprises recovering a material from the reaction having an average particle size of less than about 50 nm wherein the material, upon exposure to 900° C. for at least about 10 min., retains a surface area of at least about 100 m²/g.
 66. A method as in claim 54, wherein the recovering step comprises recovering a material from the reaction having an average particle size of less than about 50 nm wherein the material, upon exposure to 1100° C. for at least about 10 min., retains a surface area of at least about 100 m²/g.
 67. A method as in claim 54, wherein the recovering step comprises recovering a material from the reaction having an average particle size of less than about 50 nm wherein the material, upon exposure to 1300° C. for at least about 10 min., retains a surface area of at least about 100 m²/g.
 68. A method as in claim 54, wherein the recovering step comprises recovering a material from the reaction having an average particle size of less than about 50 nm wherein the material, upon exposure to 1300° C. for at least about 2 hours, retains a surface area of at least about 100 m²/g.
 69. A method as in claim 59, wherein the surfactant is a non-ionic surfactant.
 70. A method as in claim 59, wherein the particle is coated with a metal oxide layer.
 71. A method as in claim 59, wherein the particle is a metal oxide particle. 